This invention relates to sputtering targets for thin film products. Such targets can be planar or cylindrical, and have a backing body in the form of a plate or tube—the so-called support—to which the ceramic or metallic target material is bonded.
During sputtering, targets are exposed to ion bombardment and by consequence high thermal loads, and the backing body is therefore water-cooled. Large sputtering targets generally consist of a backing tube or backing plate on which one or more target segments are bonded, i.e. they are attached to the backing body by a bonding material which can be a metal—or metal alloy—having a low melting temperature, also called a metallic solder, or any other kind of electrically and thermally conductive adhesive, such as a filled elastomer.
The function of the support is to provide for electrical power transfer, mechanical strength, and heat transfer to the cooling water, and it allows the target to be mounted in the sputtering source. As the target segments—in the form of plates or cylinders—are often bonded on the support by soldering, there is in most cases a mismatch in coefficients of thermal expansion between the target material and the support materials, which results in thermal stresses on the solder layer and on the interfaces between target segment and solder as well as between solder and backing body, especially during cooling down after bonding or soldering of the target, but also during usage of the target in the sputtering process. In addition there can be a volume contraction of the solder when it solidifies, which also leads to interface contraction stress.
The cumulative thermal and contraction stress often leads to strong and uncontrolled delamination of the solder layer from either the backing body or the target segment material or from both. Where delamination over larger surface areas occurs, heat dissipation from the target segment to the support is minimal, which leads to local overheating, provoking more uneven thermal stress and eventually cracking of the target segments during sputtering.
In an attempt to overcome the problem cited before, in US2009/0152108 the adhesive bonding of target to backing body is replaced by a plastically deformable compensation means that compensate for the deformation of both target and backing material. In one embodiment, the compensation means are formed by locally limited solder bridges partially connecting the target and the carrier member to one another. In the carrier member, cut-outs are provided in the manner of a spiral or ring grooves, where the solder resides before the bonding takes place. Upon heating and liquifying of the solder, the target and carrier member are rotated such that the liquid solder moves out of the cut-outs by centrifugal forces to fill the gaps between target and carrier member and form solder bridges, but not at the height of the cut-outs. The solution given to the stress problem in this patent application is still rather complex.
In view of the above cited delamination problem, there is need for a more simple and reliable bonding system.